1. Field of the Invention
The present invention relates to an oscillation measurement method and a frequency measurement apparatus and in particular, to an oscillation measurement method and a frequency measurement apparatus for measuring an oscillation state of an object by using a self-mixing type laser Doppler oscillation meter.
The present invention can be applied to an inspection and calibration apparatus for an oscillation generating apparatus and to apparatus of an abnormal oscillation detection in a power apparatus operating for a long period of time. As the inspection/calibration apparatus, the present invention can be applied for inspection of a frequency of, for example, a crystal oscillator and an ultrasonic oscillator and for calibration of a function generator. Moreover, as the abnormal oscillation detection apparatus, the present invention can be applied to a detection apparatus for a frequency deviation caused by undesirable resonance in a semiconductor manufacturing equipment which utilizing a high frequency oscillation as well as a defect in a tool such as a drill.
The invention can also be applied for purpose of oscillation analysis. More specifically, analysis of engine oscillation, analysis of vehicle body oscillation, analysis of noise in a vehicle, analysis of muffler oscillation, and the like. The invention can also be applied to various production fields. The invention can also be applied for detection of oscillation of a plant using a motor, leak analysis of a water pipe and a gas pipe for maintenance. Furthermore, the invention can be applied to determine sugar content in a large-size fruit such as a watermelon through a hitting sound. Here, the xe2x80x9cobjectxe2x80x9d to be measured has a wide range from crystal oscillators to watermelons.
2. Description of the Related Art
Conventionally, as means for measuring a frequency of a vibrating object in non-contact way, there is a method for obtaining a frequency by using a laser displacement meter utilizing the trigonometrical survey.
However, in the aforementioned conventional example, it is impossible to detect a displacement in a short period of time because of the sampling time of the displacement meter and has a problem that an error is caused in the measured oscillation cycle because of the sampling time. This becomes especially remarkable when the object oscillation becomes greater to reduce the difference between the oscillation cycle and the sampling time. That is, a great error is caused unless the sampling cycle is more than twice the maximum oscillation cycle (sampling theorem). Moreover, since the trigonometric survey is used, there is a need of using a large sensor head whose position and direction should be adjusted so that a reflected beam will not be cut off. Accordingly, this cannot be used for measurement in a small space.
It is therefore an object of the present invention to provide an oscillation measurement method and a frequency measurement apparatus capable of measuring a small oscillation of an object with a high accuracy.
The oscillation measuring method according to the present invention comprises: a laser beam emission step for emitting a laser beam to an object to be measured; a return beam reception step for receiving a return beam reflected by the object and having an object Doppler frequency according to a velocity of the object; a self-mixing step for mixing the return beam having the Doppler frequency with beam emitted itself upon reception of the return beam and generating a self-frequency according to a resonator change during a time from the emission to the reception of the return beam, so as to generate a beat wave containing the object Doppler frequency superposed with the self-frequency; and an oscillation information output step for outputting the beat wave or information obtained from signal processing of the beat wave as the object oscillation information.
The oscillation measuring method according to the present invention measures an oscillation utilizing self-mixing type laser Doppler effect. A return beam having a Doppler frequency according to a velocity change of an object to be measured is self-mixed with an emitted beam generating a self-frequency of the resonator itself. The self-frequency is generated, for example, by a method for driving with a drive current of a sinusoidal waveform having a frequency in the proximity to the self-frequency and a method for physically oscillating the laser block having the resonator. When the resonator is driven by a drive current of a sinusoidal waveform, the oscillated beam has a wavelength changing according to the drive current value, causing a difference between the emitted beam and the return beam. This difference generates a beat wave of the self-frequency in the resonator. When the laser block having the resonator is physically oscillated, a Doppler frequency is generated by the resonator velocity and the velocity of the object to be measured. The Doppler frequency of the self-frequency by the resonator velocity is superposed by the Doppler frequency of the object to be measured. The self-frequency can be considered to be an imaginary velocity of the resonator. In other words, two Doppler frequencies based on two velocity values are mixed. Here, a phrase xe2x80x9chaving a Doppler frequencyxe2x80x9d means that a returned beam has a frequency shifted by the Doppler effect and has the shifted component as the Doppler frequency.
When the self-frequency change is in the proximity to the Doppler frequency change, a beat wave is generated according to a difference between the two frequencies. This beat wave has an envelope having a longer cycle as compared to the cycle of the Doppler frequency. Since the frequency of the envelope is a difference between the two frequencies, for example, in a preferred embodiment, it is possible to use this envelope frequency to obtain a difference between a frequency of the object to be measured and a frequency of an imaginary velocity of the resonator. Since the envelope frequency is lower than the Doppler frequency of the object to be measured, the accuracy in obtaining the envelope frequency value is higher than the accuracy in obtaining the Doppler frequency. Accordingly, according to the present invention, it is possible to improve the effective figures of the detectable miunute frequency difference without increasing the accuracy of the A/D converter or the like.
In the laser emission step, a laser beam is emitted to the object to be measured. The laser beam is scattered and reflected by the surface of the object to change its frequency according to the velocity of the object. In the return beam reception step, this return beam is received. In the self-mixing step this return beam is self-mixed in the resonator, with an emitted beam (oscillated beam) emitted upon reception of the return beam. The emitted beam emitted upon reception of the return beam generates a self-frequency according to a resonated change during a period from the laser beam emission to the laser beam reception. Accordingly, in the present invention, a newly emitted beam generating a self-frequency is self-mixed with the return beam having the Doppler frequency of the object to be measured. The oscillation information output step outputs as the oscillation information of the object to be measured a beat wave generated by the mixture of the object Doppler frequency and the self-frequency or information of the beat wave subjected to a signal processing.
This beat wave is useful for detecting a change of the oscillation state of the object to be measured. Especially when the self-frequency is in the proximity to the object oscillation frequency, it is possible to obtain a beat wave characterized in the envelope waveform. From this envelope frequency, it is possible to calculate a fine frequency difference between the modulation-frequency and the object frequency.
Moreover, when the envelope value change is below a predetermined value, it is possible to determine that the self-frequency is matched with the object frequency in valid digits. Accordingly, it is possible to calculate a frequency of the object by successively changing the self-frequency to detect an envelope value change, i.e., by synchronizing the self-frequency with the object frequency.
Moreover, the frequency measuring apparatus according to the present invention comprises: a laser resonator for oscillating a laser beam and self-mixing the laser beam reflected by an object to be measured and returning as a return beam with a laser beam emitted upon reception of the return beam; a laser drive block for driving the laser resonator with a laser drive current of a sinusoidal wave; a laser block for emitting a laser beam oscillated with a wavelength according to the drive current in the laser resonator to the object to be measured and outputting a beat wave obtained from self-mixture in the resonator, of a return beam from the object with an emitted beam oscillated with a wavelength according to a drive current in the laser resonator upon reception of the return beam; and a signal processing block for performing signal processing of the beat wave output from the laser block and outputting a processed result as an oscillation information, wherein the signal processing block includes a fine frequency difference calculation function for calculating an oscillation frequency of the object according to a frequency change of the beat wave.
In this frequency measuring apparatus, the laser is driven by the laser drive with a laser drive current of a sinusoidal wave. Then, in the resonator, the wavelength of the emitted beam is changed according to the sinusoidal wave. When wavelength of the emitted beam in the resonator is changed, a beat frequency is generated between the emitted beam and the return beam corresponding to turn-around of flight time. When the laser drive current is linearly increased, for example, the beat frequency is constant if the object to be measured is in a still state. In this invention, since the laser drive current is input as a sinusoidal waveform, the beat frequency according to the oscillated wavelength change is changed with a cycle of the laser drive current waveform. This beat frequency change is the self-frequency. Accordingly, in the frequency measuring apparatus according to the present invention, the wavelength change of the emitted beam (oscillated beam) is assumed to be an imaginary velocity of the resonator, and the corresponding beat frequency is used as the self-frequency. Moreover, the Doppler frequency is changed according to a velocity change of the object to be measured. When the object is oscillating, the velocity is 0 at the oscillation return position where the velocity direction is reversed. If a sinusoidal wave oscillation is assumed, then the object velocity is changed with a sinusoidal waveform. In this case, the Doppler frequency is also changed with a sinusoidal wave.
When the laser drive is performed with a sinusoidal wave and the object is moving, the return beam has an object Doppler frequency. Then, in the beat wave generated by the self-mixing in the resonator, the self-frequency is superposed by the object frequency (Doppler frequency). The cycle change of this beat wave is based on a fine frequency difference between the self-frequency and the object frequency. Accordingly, by observing the cycle change of this beat wave, it is possible to measure a difference between the object to be measured and the self-frequency as well as a frequency of the object and an oscillation cycle change of the object.